2This book is a collection of chapters written by different authors to fulfill several goals. In addition to providing a state-of-the-art review of the field of polymeric gas separation membranes, it was conceived as a specialized textbook for students or those beginning to work in the area. It is also intended for the professional who is experienced in some areas of the field but wants to gain broader knowledge. Finally, it is our hope that this book will help guide the future course of research and commercial practice in this field. To achieve these goals, we felt it was necessary to include a balance of fundamentals, current applications, and consideration of future possibilities. The chapters have been arranged to achieve a logical flow that might be useful for students who wish to read from cover to cover; however, each chapter has been designed to stand alone well enough that the professional can read them in any order depending on interests or needs. Naturally this means a certain amount of repetition. This introductory chapter is intended to give some background and perspective to aid understanding of what follows.